The Streak of Minerals
The streak of a mineral is the color of the powder produced when it is dragged across an un-weathered surface, typically a piece of unglazed porcelain known as a "streak plate." Unlike the mineral's surface color, which can vary due to weathering or impurities, the streak reveals the true inherent color of the mineral in its powdered form.
If no streak appears, then the mineral's streak is said to be white or colorless. Streak is especially important for identifying opaque and colored materials and is less useful for silicate minerals, most whereof have a white streak and are too hard to easily powder.
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| Examples of Mineral Streaks |
The apparent color of a mineral can greatly vary because of trace impurities or a disturbed macroscopic crystal structure. Small amounts of an impurity that strongly absorbs a particular wavelength can radically change the wavelengths of light that the specimen reflects, thereby changing the apparent color. Dragging the specimen to produce a streak breaks into randomly oriented microscopic crystals, reducing the small impurities' effect.
The surface whereover the mineral is dragged is called a "streak plate" and generally is made from unglazed porcelain tile. The unglazed underside of a porcelain bowl or vase or the back of a glazed tile also works. Sometimes a streak is more easily or accurately described by comparing it with the "streak" made by another streak plate.
Why is the streak important
More consistent than color: A mineral's color can vary greatly depending on factors like impurities, weathering, and lighting. The streak, however, tends to be more consistent and often reveals the true underlying color of the mineral.
Diagnostic tool: For minerals with variable colors, the streak can be a key distinguishing feature. For example, both pyrite and chalcopyrite can appear yellow or brown, but pyrite has a black streak, while chalcopyrite has a greenish-black streak.
Useful for opaque and colored minerals: For opaque or heavily colored minerals, where seeing through to the true color is impossible, the streak becomes even more vital in identification.
The streak can also help differentiate between minerals with similar appearances, such as quartz and calcite, which have different streak colors (white for quartz and colorless for calcite).
How to perform the streak test
- Choose a clean, unglazed porcelain plate or another hard, smooth surface.
- Rub the mineral firmly against the plate in a straight line.
- Observe the color of the powder left behind. This is the streak.
Minerals Streak Examples
The streak test is a valuable tool for mineral identification, and a single mineral can offer various clues depending on its composition and structure. Here are some examples of different minerals and their unique streaks:
Hematite - Red to reddish-brown streak.
Magnetite - Black streak.
Pyrite - Greenish-black or brown-black streak.
Chalcopyrite - Greenish-black streak.
Galena - Lead-gray streak.
Sphalerite - Brown to light yellow or even white streak.
Limonite - Yellowish-brown streak.
Malachite - Light green streak.
Azurite - Light blue streak.
Fluorite - White streak (though the mineral itself can be various colors).
Quartz - White streak (useful as quartz is colorless or white in its pure form).
Calcite - White streak, even though calcite can be many colors.
Cinnabar - Scarlet red streak.
Graphite - Black to dark gray streak.
Molybdenite - Bluish-gray streak.
Talc - White, apple-green, or gray streak.
Gypsum - White streak.
Biotite - Colorless or white streak, despite being dark brown or black in color.
Chalcanthite - Colorless to white streak.
Corundum - White streak, regardless of its color variety like ruby or sapphire.
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| Minerals streaks |
White Streak:
- Calcite: Commonly found in sedimentary rocks, calcite has a white to light grey streak.
- Quartz: This abundant mineral, found in various forms like amethyst and citrine, always leaves a white streak.
- Feldspar: A group of common rock-forming minerals, different types of feldspar have white streaks, like orthoclase and plagioclase.
Black Streak:
- Galena: This lead sulfide mineral has a characteristic metallic grey to black streak.
- Graphite: This soft, black mineral leaves a black streak that can be used to write on paper.
- Magnetite: This black, magnetic mineral has a black streak and is attracted to magnets.
Red Streak:
- Cinnabar: This mercury sulfide mineral is bright red and has a red streak, making it easily identifiable.
- Realgar: This arsenic sulfide mineral is orange-red and has a red to orange-red streak.
Luster of Minerals
The Mohs scale of mineral Hardness
The Complete Classification of Minerals
Crystal Structure and Crystal System